1. Field of the Invention
The present invention relates to a diffraction element, objective lens unit, optical pickup, optical disc apparatus and design method for a diffraction element, and is preferably applied to an optical disc device, for example.
2. Description of Related Art
In recent years, there is an optical disc device that supports a plurality of types of optical discs: “Blu-ray Disc (Registered Trademark)” (BD) along with well-known formats such as Compact Disc (CD) and Digital Versatile Disc (DVD).
The optical disc device chooses one of the following types of optical beam in accordance with the format of an optical disc inserted: approximately 780 nm wavelength of optical beam for CD, around 660 nm wavelength of optical beam for DVD or about 405 nm wavelength of optical beam for BD.
By the way, it is desirable that the optical disc device be equipped with an objective lens that supports the three types of wavelengths to be simplified and downsized: The objective lens is installed in an optical pickup that emits an optical beam to an optical disc.
However, different wavelengths of optical beams are used for CD, DVD and BD formats. In addition, their protection layers are different in thickness (or the distances from lower surfaces of the optical discs to their signal recording surfaces are different). Moreover, their numerical apertures for objective lens are different.
Accordingly, it is difficult to design the objective lens that supports the three types of wavelengths. It is difficult to obtain one with good characteristics because of lower transmission efficiency and aberration of optical beams and the like.
Therefore, correcting the aberration of optical beams is one way to cope with the above problem: a diffraction element that selectively diffracts particular wavelengths of optical beams may be used along with the objective lens.
For instance, there is an optical disc device equipped with a diffraction element that only diffracts an optical beam for CD while optical beams for DVD and BD are not diffracted: the diffraction element has two different refraction indexes of substances and a step-like diffraction pattern formed in between these substances (see Jpn. Pat. Laid-open Publication No. 2005-302270 (Pages 15-19 and FIG. 12), for example). The optical beams for CD, DVD and BD are also referred to as a “CD-type optical beam”, “DVD-type optical beam” and “BD-type optical beam”, respectively.